1. Technical Field
This invention relates in general to microelectromechanical system (MEMS) devices and, more particularly, to an electrostatically driven MEMS device.
2. Description of the Related Art
Microelectromechanical systems (MEMS) devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching, which have been developed for the fabrication of integrated circuits. Digital micromirror devices (DMDs), sometimes referred to as deformable micromirror devices, are a type of MEMS device used in projection displays by controlling light through reflection. Other types of MEMS devices include accelerometers, pressure and flow sensors, and gears and motors.
A conventional DMD 100 is illustrated in FIG. 1. As shown, the DMD 100 is constructed of three metal layers: a top layer 102, a middle layer 104, and a bottom layer 106. The three metal layers are situated over an integrated circuit (not shown), which provides electrical commands and signals. The top layer 102 includes a pixel mirror 108 that resides over the middle layer 104 supported via a mirror support post 110. The middle layer 104, in turn, resides over the bottom layer 106 supported by four hinge support posts 112. The mirror support post 110 of the top layer 102 is attached to a yoke 114. As the yoke 114 rotates on its torsion hinges 118, it drives the mirror support post 110 to rotate and tilt accordingly. Consequently, as the mirror support post 110 rotates and tilts, it dictates the angle, direction, and magnitude that the pixel mirror 108 will rotate and tilt. The yoke 114, in essence, controls the pixel mirror 108 by this relay effect.
One problem associated with a conventional MEMS device, such as the DMD 100, is “stiction”, which occurs when the yoke 114 rotates on the torsion hinges 118 and the yoke landing tips 116 come in physical contact with landing sites 120 located within the underlying bottom layer 106. In some cases, when surface adhesion forces are high enough, the yoke landing tips 116 may stick to the landing sites 120 in the underlying bottom layer 106, and thereby adversely affect the response time of the pixel mirror 108 and the overall device performance. In other cases, the landing tips 116 may adhere to the landing sites 120 and remain stuck if an applied mechanical restoring force is not strong enough to overcome the existing surface adhesion forces. The pixel mirror 108 will then be considered permanently defective because it will remain fixated at only one angle.
Stiction has heretofore been addressed by applying lubrication or passivation layers to the yoke landing tips 116 and the landing sites 120 in the hopes of making these metal surfaces slippery enough to minimize sticking. In addition, reset electronics 122 have been employed to pump additional electrical energy into the yoke 114 in order to help it break free from the constraining surface adhesion forces between the yoke landing tips 116 and the landing sites 120. These techniques require extra fabrication processes and additional cost.
Therefore, a need has arisen for a MEMS device which does not need special fabrication to overcome stiction forces.